1. Field of Invention
The present invention relates to an apparatus and method for stimulating a subterranean formation(s), and more particularly, to such an apparatus and method wherein one or more shaped charges are aimed axially at one or more elongated propellant members that are positioned in a subterranean well penetrating a subterranean formation of interest.
2. Description of Related Art
Individual lengths of relatively large diameter metal tubulars are secured together to form a casing string which is positioned within a subterranean well bore to increase the integrity of the well bore and provide a path for producing fluids to the surface. Conventionally, the casing is cemented to the well bore face and subsequently perforated by detonating shaped explosive charges. These perforations extend through the casing and cement a short distance into the formation. In certain instances, it is desirable to conduct such perforating operations with the pressure in the well being overbalanced with respect to the formation pressure. Under certain overbalanced conditions, the well pressure exceeds the pressure at which the formation will fracture, and therefore, hydraulic fracturing occurs in the vicinity of the perforations. As an example, the perforations may penetrate several inches into the formation, and the fracture network may extend several feet into the formation. Thus, an enlarged conduit can be created for fluid flow between the formation and the well, and well productivity may be significantly increased by deliberately inducing fractures at the perforations.
Gas generating propellants have been utilized in lieu of hydraulic fracturing techniques as a more cost effective manner to create and propagate fractures in a subterranean formation. In accordance with conventional propellant stimulation techniques, a propellant is ignited to pressurize the perforated subterranean interval either simultaneous with or after the perforating step so as to propagate fractures therein. One propellant stimulation tool consists of a cast cylinder of solid rocket propellant having a central ignition system which consists of a detonator cord positioned within a bore formed in the center of the cylinder of propellant. The bore may be provided with a thin walled aluminum or cardboard carrier or tube to assist in insertion of the detonator cord through the bore and the carrier having a plurality of apertures formed therein so that the detonator cord when ignited causes the propellant to ignite initially at each aperture. In this manner, the propellant is caused to burn in controlled, uniform manner.
These propellant stimulation tools require two independent conditions for ignition. The propellant must be subjected to a relatively high pressure, such as at least about 500 psi, and an ignition means must be fired. The ignition means conventionally consists of the detonator cord that in turn is ignited by means of conventional electric or percussion detonators. Often several of these propellant stimulation tools containing detonation cord are connected together in series to cover the length of a subterranean zone to be stimulated. As connected in series, a booster or ballistic transfer is located in a tandem sub that links the detonating cords in the propellant stimulation tools located above and below the tandem sub. This booster or ballistic transfer is needed to amplify the energy generated by the detonating cord at the end of a propellant stimulation tool to the level needed to ignite the detonating cord of the next propellant stimulation tool in the series.
However, these conventional propellant stimulation tools need to be manufactured to close tolerances to permit the detonating cord and boosters to effectively initiate an explosive transfer between tools. Further, the ends of each tool need to be sealed to ensure a fluid tight environment exists around the detonating cord to ensure detonation thereof. Such conventional propellant stimulation tools have relatively high manufacturing and component part costs and require operating personnel to have a relatively high level of skill. Thus, none of these prior art devices which utilized propellants in stimulation tools have provided completely satisfactory results in well bores of varying diameters or a repeatable and reliable propellant burn in a discrete or controlled pattern. In view of this, a need exists for a propellant stimulation tool which eliminates the need for a detonating cord for ignition while providing for more reliable detonation of the propellant, decreased manufacturing and components costs and a decreased level of skill for operating personnel.